Pneumatic tires for vehicles are a fundamental component which generate forces that control vehicle motion. It has been recognized for some time that valuable information can be derived as to tire performance and design with the capability of measuring the forces and moments exerted by a road surface on a tire as steering, cambering, and braking take place. It is necessary that these forces and moments be measured rather than calculated because current computational mechanics technology is inadequate for the purpose of computing the forces and moments exerted by the road on the tire with the accuracy required for engineering purposes.
Early indoor systems for making tire force and moment measurement employed a flat roadway surface in the form of a planar bed or plank or employed round, convex, simulated roadways which were engaged by a tire on an external road wheel. Later, internal road wheel machines were employed wherein a tire engaged the internal surface of a round, concave, simulated roadway. All of these systems had significant limitations, such as the fact that the forces developed on a round or curved surface engaged by a tire are not equivalent to those which are developed on a flat surface like an actual road surface.
In order to achieve the accuracy advantages which can be realized only by a flat road bed, more recent systems have employed a continuous flat-surface machine consisting of an endless belt engaging two spaced rollers to provide a flat test surface there between. Improvements in machines of this type with sophisticated controls have reached a point that it is possible to monitor vehicular behavior during loss of control and in the simulation of anti-lock brake and traction control systems. Obtaining the requisite data for this type of analysis requires simultaneous and interactive control of slip angle, slip angle rate, inclination angle normal force, roadway velocity, spindle speed, and spindle speed rate. A tire test system known as the "Flat Track II" manufactured by MTS Systems Corp. incorporates the basic capabilities for obtaining this data, such that it is possible to make a meaningful analysis of the performance of a test tire under controlled operating conditions.
While the "Flat Track II" provides a high degree of accuracy with respect to most types of settings and test results, a problem has been observed with respect to residual pull testing. Residual pull testing requires a highly accurate determination of zero slip angle. In addition, the repeatability of accurate determination of zero slip angle is necessary for comparing results of long-term experiments, which may be conducted over a substantial period of time. In such instances, it is required that a highly accurate and reproducible determination of zero slip angle must be possible, despite the necessity of recalibration of zero slip angle, which may be required during the conduct of certain types of tests, because of electronic problems in the system, or due to other variables which necessitate recalibration of slip angle.
Zero slip angle contemplates the condition where the tire being tested, the wheel, and the face of the spindle on which the wheel is mounted are all precisely aligned with the direction of travel of the stainless steel belt serving as the roadway. The calibration fixture supplied with the "Fast Track 2" machine endeavors to align the spindle face with a calibration bar which is attached to the stainless steel belt and which in turn is aligned with the movement of a point on the belt as it is displaced a short distance between the supporting drums. Because of the attachment of the calibration fixture to the lateral edges of the belt, variations in the belt construction, and perhaps other factors, slip angle recalibration can be effected only to an accuracy of approximately plus or minus 0.025.degree.. It has been found that an accuracy of this magnitude is inadequate for meaningful calculation in residual pull tests.
In addition to the lack of sufficient accuracy for some tests, recalibration employing the standard fixture attached to the belt is a relatively slow and meticulous operation to effect attachment of the fixture supporting structure to the belt and to otherwise complete the calibration operation before subsequent removal of the fixture mounting structure. Thus, the time and inconvenience of recalibration employing the standard fixture and the lack of sufficient accuracy have impeded successful calculations during residual pull testing and other operations dependent upon highly accurate and reproducible slip angle calibration.